float test for specific gravity


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Can someone please tell me the float test for specific gravity. I haven't done it for several years and I can't seem to find my papers on it. I know you submerge lengthwise in water and measure the amount that sticks out but that's about all I can remember. Any help would be great.

Thanks, Berl

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What I'm talking about is filling a bucket with water and holding one end and let the other end down in the water until it floats and measuring how much is sticking out when it floats. I think you divide the float amount by the submerged amount to get the SG. Like I said it's been several years sense I've done this and I don't trust my memory.

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I do both the measured dimensions method and float method. The float method works best for a billet (wedge) that is symmetrical along the long axis. The ends are cut perpendicular.  In other words, a well-split (quartered) billet.

 

The trick is to hold the billet vertical in the water to the point where is just starts to float. Mark that depth.  Reverse the billet. Put the other end in down to where the billet start to float. Mark that. The specific gravity is the dunked depth divided by the billet length. Take the average of both measurement to reduce errors. Example: if the billet is 400mm long and sinks 135mm, the specific gravity is 0.34 . If the billet is well cut the other depth will be also 135mm or close to it.

 

I made a little jig to hold the billet vertical as I slowly lower the billet. It consists of a pair of angle iron bars sitting in a pail of water. 

 

 

post-6615-0-06428900-1398715708_thumb.jpg

 

There is no water in the pail for this photo. I find that the flotation method is faster than the measured dimensions for ~3 or more wedges. I set this up whenever I need to measure a bunch of billets.

 

Don't get scared off about getting the wood wet. It won't warp and will be dry in a few days unless you let the wood sit in the water for a few days and live in a humid climate like Houston, TX. I once did.  :(

 

 

 

 

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I do both the measured dimensions method and float method. The float method works best for a billet (wedge) that is symmetrical along the long axis. The ends are cut perpendicular.  In other words, a well-split (quartered) billet.

 

The trick is to hold the billet vertical in the water to the point where is just starts to float. Mark that depth.  Reverse the billet. Put the other end in down to where the billet start to float. Mark that. The specific gravity is the dunked depth divided by the billet length. Take the average of both measurement to reduce errors. Example: if the billet is 400mm long and sinks 135mm, the specific gravity is 0.34 . If the billet is well cut the other depth will be also 135mm or close to it.

 

I made a little jig to hold the billet vertical as I slowly lower the billet. It consists of a pair of angle iron bars sitting in a pail of water. 

 

 

attachicon.gifSpecific Gravity Fixture.jpg

 

There is no water in the pail for this photo. I find that the flotation method is faster than the measured dimensions for ~3 or more wedges. I set this up whenever I need to measure a bunch of billets.

 

Don't get scared off about getting the wood wet. It won't warp and will be dry in a few days unless you let the wood sit in the water for a few days and live in a humid climate like Houston, TX. I once did.  :(

You the man. Thank you Mike.
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I don't like getting the wood wet, so I always use the weight and dimension method.  If you measure the width and thickness at the middle of the wedge, it's usually an average of what it is at the ends.

I don't see a need for super precision in determining density, and the measurement method I think is well within 5% accuracy.

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Don you may be right, but I think that Peter's method is more accurate and no more difficult even for a bad mathematician like myself. Having said that, Michael's water method also works fine and there is absolutely no problem with the wood getting wet. It will be back to normal in a hour or two at the most. The real problem with the water method is persuading your wood dealer to let you do it.

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You can get plasitc corners, used by the drywall guys, to hold the wood vertical.  

If the sides are pretty flat the average of two ends will be accurate.

 

P.S. In houston the air-conditioner dries the air very nicely.  Just do not do it outside!

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I don't like getting the wood wet, so I always use the weight and dimension method.  If you measure the width and thickness at the middle of the wedge, it's usually an average of what it is at the ends.

I don't see a need for super precision in determining density, and the measurement method I think is well within 5% accuracy.

with your weight and dimension method ( or whoever first figured it out) what is the math. peter gives the volume. thanks  from the (mathematically slow) 

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Working with density in grams/cubic centimeter, you want weight (grams) divided by volume (cc).

 

The way I do it:

 

Weight / (L x W x (T1 + T2)/2)

 

Where T1 and T2 are the thickest and the thinnest edges of the wedge.  This only works if the wedge is a pure wedge, and reasonably flat faces.

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Another thing to observe is density changes between seasons. The same piece of wood can vary 5 - 10 % in density between winter and summer. This is why I think it's best to measure and weigh for calculating density, taking humidity/moisture content in the wood into accont.

 

The top I'm going to use for my next violin has already changed from 0,50 to 0,48, at normal condition (6% EMC) it will be ~0,46.

Of course you can argue that one does not need to bee that accurate - but then again why measure at all.

 

My opinion is If you measure, do it properly, otherwise you are collecting bad, non comparable data.

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Can someone please tell me the float test for specific gravity. I haven't done it for several years and I can't seem to find my papers on it. I know you submerge lengthwise in water and measure the amount that sticks out but that's about all I can remember. Any help would be great.

Thanks, Berl

Hi,

If you want to be absolutely precise, float your wedge in a water container and measure the water it replaces, after this dunk it fully underwater and measure the water it replaces, that will give you the exact wolume of any shape wood. The other posters gave you the maths to work it out.

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...float your wedge in a water container and measure the water it replaces,

 

It is easier and more accurate to just weigh the wood for this step.  It will displace its weight in water, and since water density is 1g/cc, it will displace as many cc's of water as it weighs in grams.

 

However, this does bring up another idea for finding the accurate volume of the wood, which is the main problem.  Fill a container to the top with water.  Dunk the wood completely in the water, carefully collecting all the water that is displaced and spills out.  Weigh the water collected.  The density of the wood will then be:  weight of wood / weight of water.

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I don't like getting the wood wet, so I always use the weight and dimension method. If you measure the width and thickness at the middle of the wedge, it's usually an average of what it is at the ends.

I don't see a need for super precision in determining density, and the measurement method I think is well within 5% accuracy.

I usually measure the three dimensions but lately I'm more inclined to do as Don does. Maybe because I'm more experienced understanding the density's import or maybe I'm getting lazy. Anyways, generally I don't think it is especially important to know, as a practical matter, anything more than low/medium/high. Ditto goes for the speed of sound.

And I don't like sticking my dry wood in water either.

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In one respect, this information is very interesting.

 

... just for my own curiosity, once you have a figure or measurement of the woods density... once you get it put into a figure, a specific number, well, what do you do with that figure? Exactly what do you need that measurement for?

Do you use it to compute the maximum or minimum thickness of the plate, once the billet is cut and joined, and the plate outline is cut, and the plate is thicknessed - ?

Or, perhaps it is then used to calculate the finished plates weight ?

Or, is it used to find the "lightest or the heaviest" per cubic whatever, wood that you have available?

Is this technique used for both Spruce and Maple - either one?

 

I do understand why someone would try to get this data for pernambuco wood in bow making, but I have not understood why or what someone would do with this figure in or for violin making wood.

 

Just curious.

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Craig, on the value of the knowledge,and what to do with it,... I see it as a landmark , in sailing triangulation is used to determine exact location ,a light house ,a smoke stack and a cliff side ,  knowing the original value is say 45 and you have 34 , a maker might then leave a bit of meat to achieve similar working qualities , also the ability to select different styles,measurements and using the same qualities of wood ,to see variety of tonal effect ,impossible if the deck is shuffled in between every game.  I'm sure the stand alone data ,means nothing in and of itself , but when compared or triangulated against other known values,like speed of sound , cross grain stiffness, arch height,arch form, grads weight  ,it can serve as a tool to help achieve consistent tonal qualities.not a magic bullet .... 

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